In belt transmissions, it is problematic when the belt part, which transmits torques between two rotating elements, has slippage, that is, the belt part moves without grasping. In this case, considerable damage can occur to the belt transmission, especially, considerable wear of the belt part can occur. At times, the transmission can be destroyed when a slippage of the belt part occurs which is too great.
The above problem is especially relevant for CVT transmissions. CVT transmissions are continuously operating transmissions (CVT=continuously variable transmission). The CVT transmission includes essentially two conical wheel pairs and a belt part designed, for example, as a thrust member band. One of the conical wheel pairs is connected to a drive, for example. of an internal combustion engine while the other conical wheel pair is connected to an output. To adjust the transmission ratio of a CVT transmission and the tension of the belt part, the drive conical wheel pair and the output conical wheel pair essentially include each an axially fixed and an axially movable conical wheel. The drive conical wheel pair is characterized also as a drive disc or primary disc. The output conical wheel pair is also characterized as an output disc or a secondary disc. The pressing of the axially movable conical wheels against the belt part takes place, in general, via a build up of hydraulic pressure, for example, via a pump. The desired transmission ratio of the CVT transmission and the required tension of the belt part can be adjusted with a suitable selection of the contact pressures. The pump for the hydraulic drive of the conical wheels can, for example, be driven by the internal combustion engine. For example, a torque converter and a planetary set with clutches for forward and rearward travel can be provided for the force transmission from the internal combustion engine to the drive conical wheel pair. By increasing the contact pressure, a slippage of the belt part can, as a rule, be avoided; however, an increased contact pressure leads to an unwanted increase of the losses, for example, pump losses. The control (open loop and/or closed loop) of the pressing force must therefore find an optimum between minimal pressure losses on the one hand and the slippage which is to be avoided on the other hand. In this connection, it is already known to build up a pressure reserve in order to avoid slippage of the belt part, which, for example, can be caused by the dynamic and disturbance in-couplings which are dependent upon the roadway characteristics. In each case, it is necessary to timely detect a slippage to be able to avoid an unwanted occurrence of a slippage of the belt part.